The geological periods during which coal and other materials which must be mined underground were formed were periods also conductive to shale, sandstone and limestone formation in contact therewith and sometimes interlayered with such materials. These rock structure materials vary greatly from site to site and many are mechanically weak and/or fragile. It is well known that mining under a roof of weak rock is frought with hazards. Hundreds of men are killed each year in mine cave-ins.
With present inspection methods, very little is known about the superstructure properties of a specific mine roof as mining progresses. Visually, only those cracks which intersect the walls or roof of the mine can be observed. Fissures, channels and other geological structure lying approximately parallel to the wall or roof of the mine are undetectable. The miner's traditional sight, sound and vibration techniques wherein the miner taps the wall or roof and uses fingertip detection are highly subjective and at the same time are useless in some roof configurations. One attempt to mechanize such inspection is disclosed in U.S. Pat. No. 2,549,076. While the teachings of said patent may be an improvement over tapping and fingertip detection which is still being used today, it has many disadvantages and is limited in its ability to facilitate its intended object. For example, the teachings of said patent require two test stations, one of which includes an area of rock of known strength.
Ultrasonic non-destruction inspection devices for inspecting or testing metal parts at frequencies in the megahertz range are known. Such devices are unadaptable to testing the walls or roof superstructure of a mine. Seismic testing of geologic structure such as continental shelves and mountain ranges is also well known and is accomplished at very low frequencies to achieve long distance penetration. For example, see U.S. Pat. Nos. 3,226,535 and 3,938,072. The low frequencies associated with such seismic testing are unadaptable for mine inspection.
There is a need for a rugged inspection apparatus having a lobed beam for inspecting geological structure behind walls and roofs in mines.